1. Field of the Invention
The present invention relates to an image processing device that executes an image process of matching two images with each other. The invention further relates to an image processing program storage medium storing an image processing program that causes a computer to operate as the image processing device.
2. Description of the Related Art
In the field of medical treatment, medical images taken of body portions of examination objects by using x-rays or the like are popularly utilized for diagnosis of disease conditions of the examination objects. With the utilization of medical images for diagnosis, the progress or the like of a disease condition of an examination object can be ascertained without exterior damage being caused on the examination object. Thus, information necessary for determining treatment policies or directions can easily be obtained.
In addition, in recent years, there have been popularly utilized apparatuses that acquire digital medical images. The apparatuses are, for example, computed radiography (CR) apparatuses that acquire digital medical images by using x-rays, computerized tomography (apparatus) apparatuses that acquire tomographic images of examination objects by using x-rays, and magnetic resonance imaging (MRI) apparatuses that acquire tomographic images of examination objects by using intense magnetic fields. Thus, digital medical images are generally utilized in lieu of medical images that have been acquired by using conventional films such as x-ray films.
With the digitization of medical images, diagnosis can be performed more effectively than in conventional cases by applying image processes to medical images. Among such image processes being applied to medical images, there is an image process called an energy subtraction (ES) process, which is one of processes having drawn attention in recent years. The ES process is an image process in which, of two medical images acquired using x-rays having levels of energy different from each other, an operation is done to subtract pixel values in the other of the medical images from pixel values of one of the medical images by allocating predetermined weights thereto. Thereby, in a body structure of an examination object captured on the former medical image, an image region in a site having a specific absorption property against the x-ray is erased. According to the ES process, a soft tissue portion image formed only of soft tissue image regions can be obtained by erasing bone image regions from the medical image, and conversely, a bone portion image formed only of bone image regions can be obtained by erasing the soft tissue image regions from the medical image. As a consequence, a disease focus hidden by bone, a disease focus existing in bone, and the like are likely to be interpreted or examined, thereby making it possible to perform effective diagnosis.
In order that the ES process is executed with high accuracy, it is preferable that two medical images match each other. In recent years, in the field of x-ray imaging, imaging using a so-called “flat panel detector” (FPD) including x-ray detection elements two-dimensionally arrayed is popularly performed. However, in imaging using such an FPD, one medical image is acquired by one imaging process, so that, when two medical images are acquired, the imaging times, i.e., times when imaging process is performed for each of two medical images, are inevitably different from each other. Hence, depending on the case, two images do not always match each other because of, for example, the posture or breathing of an examination object. In a case where two images do not match each other, when the ES process is forcibly applied thereon, inherently unpresent ghost image regions (artifacts) appear in mismatch portions between the two images, thereby hindering the examination.
Under these circumstances, techniques for performing an image process are proposed in which, of two medical image to be subjected to the ES process or the like, one of the images is warped and thereby matched with the other image (refer to Japanese Patent Application Publications Nos. 2002-32735, 2002-32764, and 2003-244542, for example).
Nevertheless, however, under the present circumstances, even by an image process such as described above, a sufficient match between two medical images cannot be obtained, and hence there frequently occur ghost image regions (artifacts) in the ES process.